GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 1996 🢒 Fleetwood 🢒 DTC 🢒 DTC P0502 Vehicle Speed Sensor (VSS) Circuit Low Input

Object Number: 42295 Size: SF
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The VSS provides vehicle speed information to the PCM. The VSS is a Permanent Magnet (PM) generator. The VSS produces a pulsing AC voltage. The AC voltage level and the number of pulses increase as the speed of the vehicle increases. The PCM converts the pulse to vehicle speed. A scan tool can display the vehicle speed.

DTC P0502 detects a low vehicle speed when there is a high engine speed in a drive gear range. DTC P0502 is a type A DTC.

Conditions for Setting the DTC
    • No MAP DTCs P0107 or P0108.
    • No TP DTCs P0122 or P0123.
    • No TFP Valve Position Switch DTC P1810.
    • Not in Park or Neutral.
    • The TP angle is greater than 17%.
    • The engine vacuum is 50-80 kPa (7.25-11.60 psi).
    • The engine speed is 3000-5500 RPM.
    • The transmission output speed is less than 250 RPM.
    • All conditions met for 3 seconds in two consecutive ignition cycles.

Action Taken When the DTC Sets
    • The PCM commands second gear only.
    • The PCM commands maximum line pressure.
    • The PCM freezes shift adapts from being updated.
    • The PCM inhibits TCC Solenoid Valve engagement.
    • The PCM illuminates the Malfunction Indicator Lamp (MIL).

Conditions for Clearing the MIL/DTC
    • The PCM turns OFF the MIL after three consecutive ignition cycles without a failure reported.
    • A scan tool can clear the DTC from the PCM history. The PCM clears the DTC from the PCM history if the vehicle completes 40 warm-up cycles without a failure reported.
    • The PCM cancels the DTC default actions when the fault no longer exists and the ignition is OFF long enough in order to power down the PCM.

Diagnostic Aids
    • A VSS DTC P0502 will set when no vehicle speed is detected at start OFF.
    • Inspect the wiring for poor electrical connections at the PCM. Look for the following conditions:
       - A bent terminal
       - A backed out terminal
       - A damaged terminal
       - Poor terminal tension
       - A chafed wire
       - A broken wire inside the insulation
    • When diagnosing for an intermittent short or open condition, massage the wiring harness while watching test equipment for changes.
    • Test circuits 400 and 401 for Electromagnetic Interferences (EMI) induced by a wiring harness incorrectly routed too near the spark plug wires.
    • Incorrect VSS calibration may set DTC P0502. Refer to service bulletins to ensure that the calibration is current.
    • Inspect for a damaged VSS or transmission rotor teeth.
    • Ensure that the VSS is correctly aligned and torqued securely to the transmission case.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Disable the traction system when performing this step. When the ignition key is cycled to the OFF position and then cycled back ON, the traction control system defaults to ON.

    2.

  2. This step verifies the integrity of the VSS.

    3.

  3. This step verifies the integrity of the VSS circuit.

    4.

  4. This step tests the resistance of the VSS.

DTC P0502 Vehicle Speed Sensor (VSS) Circuit -- Low Input (Transmission Output Speed Signal)

Step

Action

Value(s)

Yes

No

1

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check , Section 6.

2

  1. Install the Scan Tool ®.
  2. With the engine OFF, turn the ignition switch to the RUN position.

    3. Important:: Before clearing DTCs, use the scan tool in order to record Freeze Frame and Failure Records for reference. The Clear Info function will erase the data.

  3. Record the DTC Freeze Frame and Failure Records.
  4. Raise and support the drive wheels.
  5. Start the engine.
  6. Disable the traction control system.
  7. Place the transmission in any drive range.

With the drive wheels rotating, does the Trans Output Speed increase with the drive wheel speed?

--

Go to Diagnostic Aids

Go to Step 3

3

  1. Turn the ignition switch to the OFF position.
  2. Disconnect the C1 (red) PCM connector.
  3. Using the J 35616-A Connector Test Adapter Kit, connect the J 39200 DMM between harness connector terminals C1-31 and C1-32.
  4. Select the AC Volts option.
  5. Rotate the rear wheels. Ensure that the driveshaft is turning.

Is the voltage measurement greater than the specified value?

0.5V

Go to Step 4

Go to Step 6

4

Using the J 39200 DMM, measure the resistance between harness connector terminals C1-31 and C1-32.

Is the measured resistance greater than the specified value?

2700ohms

Go to Step 5

Go to Step 6

5
  1. Inspect circuit 400 (YEL) for the following conditions:
    2. • A poor connection at the PCM
    • A poor connection at the VSS
    • An open circuit
  2. Inspect circuit 401 (PPL) for the following conditions:
    3. • A poor connection at the PCM
    • A poor connection at the VSS
    • An open circuit

Refer to Troubleshooting Procedures, Section 8.

Did you find and correct a problem?

--

Go to Step 13

Go to Step 8

6

Is the resistance measured between harness connector terminals C1-31 and C1-32 less than the specified value?

1480ohms

Go to Step 7

Go to Step 8

7
  1. Inspect circuit 400 (YEL) for a short to ground.
  2. Inspect circuit 401 (PPL) for a short to ground.
  3. Inspect circuits 400 (YEL) and 401 (PPL) for a short together.

Refer to Troubleshooting Procedures, Section 8.

Did you find and correct a problem?

--

Go to Step 13

Go to Step 8

8

  1. Reconnect the C1 (red) PCM connector.
  2. Disconnect the VSS harness from the VSS.
  3. Measure the resistance between VSS terminals A and B. Use the J 39200 DMM.

Is the measured resistance within the specified range?

1480-2700ohms

Go to Step 11

Go to Step 9

9
  1. Remove the VSS.
  2. Inspect the Automatic Transmission Output (Shaft) Speed Sensor rotor for damage or misalignment.

Did you find and correct a problem?

--

Go to Step 13

Go to Step 10

10

Replace the VSS.

Refer to Speed Sensor Replacement, in On-Vehicle Service.

Is the replacement complete?

--

Go to Step 13

--

11

Inspect the PCM connector terminals for the following conditions:

    • Damaged connector pins
    • Backed out connector pins
    • Weak terminal tension
    • Repair the connectors if necessary.

Did you find and correct a problem?

--

Go to Step 13

Go to Step 12

12

Replace the PCM.

Refer to Powertrain Control Module Replacement/Programming , Section 6.

Is the replacement complete?

--

Go to Step 13

--

13
  1. After the repair is complete, select DTC.
  2. Select Clear Info.
  3. Operate the vehicle so that the transmission output speed is greater than 251 RPM for 2 seconds.
  4. Select Specific DTC. Enter DTC P0502.

Has the test run and passed?

--

System OK

Begin the diagnosis again. Go to Step 1